I’ve never seen those for sale here for what it’s worth.
I have heard of black and tan drink before which I believe is half Guinness half beer / ale but I’ve never had one or seen one ordered.
They have to know though, right? Where else would they sell it that there wouldn’t be Irish people there. I guess the mixing it half and half would piss them off from the get go anyway? “Might as well just go all the way, full asshole.”